Advanced radiotherapy requires precise imaging for treatment planning. Since the imaging and the treatment take place at different times and/or locations, patient positions must be maintained as identical as possible in the two procedures to reduce positional errors associated with internal organ displacement and deformation. Patient rotation about a vertical rotational axis is acceptable since such a rotation has no effect on the gravitational forces applied to the internal organs and thus no effect on organ displacement and deformation. Similarly, the patient may be translated relative to a radiation beam but remain in the same position relative to the gravitational field.
Treating a non-recumbent patient, e.g., in an upright position or in a forward-leaning position, may have clinical and economic advantages. However, since the CT scanners used for treatment planning are structurally limited with respect to positioning the patient in a non-recumbent position, such a position is practically not considered in the prior art and CT scanning is performed in the recumbent position. Recumbent-only imaging, e.g., CT or MRI are typically applied in a generally recumbent position, while general imaging e.g., x-ray or ultrasound may be also applied in a non-recumbent position. Typically, recumbent-only imaging provides three dimensional resolution and/or the anatomical details superior to those provided by general imaging. Therefore, treatment planning for radiotherapy is based on recumbent-only imaging.